Radar systems



June 8-, 1965 J. WATT RADAR SYSTEMS Filed April 16, 1962 RfCE/ VE@CATA/00E @A Y 3 Sheecs--Sheerl l TUBE F/G/A.

INVENTOR ATTORNEYS J. WATT 3,188,633

RADAR SYSTEMS June 8, 1965 3 Sheets-Sheet 2 Filed April 1e, 1962ATTQRNEYJ June 8, 1965 J. WATT 3,188,633

RADAR sYsTEMs Filed April 1e, 1962 s sheets-sheet s REL MOTION DISPLAYTRUE MOTION DISPLAY (c) HG2; (d)

INVENTQQ ATTORNEYS Unite Stats t 3,l88,633 Patented .inne 8, i965 tice3,1ss,63s RADAR SYSTEMS .l'ames Watt, Chelmsford, England, assigner toThe Marconi Company Limited, London, England, a British company FiledApr. 6, 1962, Ser. No. i87,479 Claims priority, application GreatBritain, Apr. 18, 196i, 13,945/61 7 Claims. (Cl. 343-120 This inventionrelates to radar system-s and though not limited exclusively to itsapplication thereto, is primarily intended lfor marine radar systems.

The problem of designing a marine radar system which will give aWatch-keeping oicer, reliably and quickly, the information necessary forsafe navigation in conditions of heavy trafc and -bad visibility has notyet been satisfactorily solved despite the different varieties of P.P.l.displays which have been proposed and used. Safe navigation involvesthat, before taking any avoiding action, the ofcer shall know whetherdanger of collision exists and whether, under the Regulations forPreventing Collisions at Sea, it is his duty to take Such action. Since,under the Regulations, risk of collision is deemed to exist if lthecompass bearing lof an approaching vessel does not appreciably change,the officer must be provided with information of the movements of the4ships concerned over the period of time necessary to determine whetheror not there is appreciable change of compass bearing and, accordingly,Whether it is his duty lto maintain his course and speed or to takeavoiding action. In theradar systems at present in general use it issought to satisfy these requirements by presenting a P.P.\I. type ofdisplay on a cathode lray tube having a screen with long aftergloweffects so that the display shows not only the momentary targetpositions but also, due to the .said afterglow effects, past historyi.e. previous target positions occupied during the period of afterglow.This dependence on afterglow has serious practical defects. In practicethe brightness of the parts of the display actually being written in atany time is much greater than the parts which persist by aftergloweffects and the latter parts are, in general, too dim for convenientaccurate use except (and not always then) under conditions of almosttotal rambient darkness. For this and other reasons it is commonpractice, and, indeed, normally necessary to safe navigation, to resortto manual plotting of observations made lof the display screen atintervals and many radar equipments include a so-called reection plotterfitted over the display tube to facilitate such plotting. The need formanual plotting constitutes a serious disadvantage not only because -of.the fact that it requires skilled man-power, but also because anundesirably long time must elapse before a plot of sufcient lengthreliably to give the required information c an be obtained. Also, thenumber of ships which can be simultaneously plotted by an officer usinga simple display as his source of information is, for obvious practicalreasons, severely limited, and an officer will, therefore, necessarilyconfine his attention .to those ships constitu-ting the most imminentthreats to safe navigation. This can be a serious cause of danger inconditions of heavy .traino Furthermore, in the case of known P.P.I.displays of the so-called true motion type, i.e. the type in which ownship representation appears in different positions on the screencorresponding to different positions occupied by lthe ship in moving-ownship representation moves Itowards the screen edge so that it may benecessary to commence plotting another ship when the own shiprepresentation is approaching the edge of the display with the resultthat a plotting operation may have to be stopped or interrupted when thedisplay is reset, or,

alternatively, it may become necessary to reset the display beforecommencing a plot.

Another defect of the usual known radar systems with only a singledisplay tube is that it is impossible, as it is often required to d-o,to change over quickly fr-om a socalled relative motion display lwithpast history in it to a true motion display with past history in it,(i.e. from a display `wi-th own ship at the centre to a display in whichown ship moves across the tube screen in accordance with ship movement)or vice versa because, when the tube is switched over from one displayto the other, it takes some time before the new display, with pasthistory in it, is built up. Of course, this defect could `be avoided byhaving two vtubes in continuous use, one giving a relative motiondisplay and the other a true motion display, but it is obviously notdesirable to have two tubes to watch, quite apart from Ithe eXtra costinvolved in providing two tubes with their associated gear, and theconsiderable danger of the observer drawing wrong conclusions regardingship movements due to his mistakenly attaching in his mind the relativemovement of lone ship to the true motion of another.

The present invention .seeks to avoid the foregoing diiculties anddisadvantages. l

According `to a feature of this invention a radar system suitable formarine use comprises means for deriving at a predetermined relativelylow periodicity, a' `succession of sets `of information, each set beingof information as to the direction and distance of radio reflectingtargets within the range of the system; a plurality of storage means;means for cyclically storing a sequence of said derived sets in saidstoragemeans with a different sequential set in each; means for takingoff, at a cyclic speed of repetition which is high relative .to theaforementioned low peri-odicity, the sets of information stored at anygiven time in said storage means; and means for utilising the sets ofinforma-tion taken off to produce a :final displayof the P.P.I. type.V

yPreferably the derived sets of information are of information as to thecompass direction and distance of radio reflecting objects.

Preferably again `signal storing means are provided for storing, duringthe deriving of the succession of sets',

of information, signals representative of Ithe amount and direction ofmovement of the radar system as Whole dur.

of the last set to be derived whereby the final display is caused to beVa true motion display with, however, the last position (at any time) ofthe radar system Vas a whole always at the centre of .the surface onwhich the lfinal display is produced. Thus the iinal display, thoughshowing true motion, is not one in which the radar system positioncreeps towards the edge of the display surface and no re-setting istherefore required. v

According to another feature of this invention a radar system suitable,for marine use includes means for producing, on the screen of a displaycathode ray tube, a reversed true motion display comprising a .pluralityof P.P.I. (plan position indicator) pictures each of informationobtained during a different space scanning operation of the radarsystem, Vthe origin of ythe picture of information obtained in the lastspace scanning operation being at a xed predetermined point (normallythe centre point) on said screen and the originV of each ofthe otherpictures displayed at any given time being spaced from said point by anamount corresponding to and Vin a direction opposite Ito the movement oftheradar system duringthe 'time interval between the two scanningsidered. j

The periodicity wat' which the succession of sets of information isderived is very much longer than Ythat at which the ,stored .sets aretaken Voff and displayed. Thus, to quote practical figures for a marineequipmen-t of the usual kind in which aA single azimuth exploration of360 is effected in 3 seconds, thus .providing a set yof information inthree seconds, each twentieth set might be stored,` successive twentiethsets being stored indifferent storage means, so that a succession ofsets is stored at a periodicity of one .per minute, and the sets storedat any given time in all the storage means lmight be taken off insequence and displayed in a cyclic time of, say, 21/2 seconds, i.e. allthe stored sets would be taken off and displayed id21/2 Y seconds.

VIt is not necessary that the time during which stored' sets ofinformation are taken off from the plurality of storage means is not thesame for all the individual storage means.

lthat storage means 'storing the most recently derived V'set than intaking from and displaying the sets of information stored in the otherstorage means. Thus, to quote a preferred and practical'embodiment,there may be three storage meansemployed to store three setsvofinformation derived during three successive periods of time (eg.duroperations Con- .television -reproducer picture tube, the displaystill being of P.P.I. type (within ythe meaning of that expression asemployed in this specification) i.e.`still being -of the type in whichthe distance and direction of a .target representation from the originof ythe display correspond respectively Yto the range and compassdirection of the target represented thereby. There are various ways inwhich a final display of what may beV termed the television P.P.I. typemay be obtained. One way isfto use camera tubes lof the storageV typeand scan the storage target structures thereof to pick-up signalstherefrom in aC- cordance with :a television line raster. However, inorder to avoid problems Vof accurate registration with three stor-Indeed, i-t is preferred tospend a longer time in taking from anddisplaying a set of information stored in age type `camera tubes, it maybe found morerconvenient to separate the functions of `scan conversionand storage. Thus, for example, the signals from only a single mastercamera tube performing the operation of yscan conversion could be-fed'via ka distributor switch to three storage devices `of the magneticdrum type, Of course, scan conversion is not a necessity to theinvention (though it is preferred) and the appara-tus may be such as tostore sets of information in polar co-ordinates and-also to take themoff and display them in polar co-ordinates.

-If it is required to produce, either all the time or at willya finaldisplay of the true motion type, there may be provided means forproducing D.C. :signals representative of mutually perpendicularcomponents (eg. Northment has theadvantage that, in the resultantdisplaythe Y last set of information to be derived and stored will bedisplayed more persistently than earlier derived and stored sets, andhence more prominently.

In one way of carrying out the invention the sets of information arederived by means of an'azimuth scanningV aerial, a pulsed radartransmitter, an echo signal receiver, what is herein termed anintermediate P.P.I. display tube of little or no afterglow, the' ray inwhich is defiected radially under the control of a range time base andcircularly at the speed of aerial scanning in azimuth (as so fardescribed the apparatus is as known and, except that the intermediatedisplay tube has little or no afterglow, like that of a normalknownradar system); a'

periodic succession of selected sets of information Yoc'curring atpredetermined relatively long intervals-ie. a chosenperiodic succession(eg. 'one in twenty) of the successive displays of the tube producedduring successive scans in azimuth--are stored. by meansincluding apluwhich is high relative to rthe low periodicity of storage, Y

to a final display tube adapted and arranged to produce a P.P.'I.display. The camera tubes may themselves effect storage of signals orseparate .storage means for storing sets of .signals produced bythecamera tubes 'may -be used. In this embodiment the ,P.P.I. display inthe intermediatev display tube will (in accordance with normal radarposition) be produced in'polar co-ordinates by a polar eoordinate.scan-i.e. by radial andcircular deections. yIt is preferred, however,that the P.P.I. displayin the final display tube be produced by atelevision scan, Le. as a picture built up -by a series of sca-nninglines as in a South and East-West) of ships motion, storage condensersfor storing said components, `and means (which maybe operable at will)for utilising signals stored in said condensers for shifting the originof the display in the final display tube in corresponding perpendicularcomponent directions ,of such .sense that the origin of the display ofthe last derived .stored 'set fof information is always at the centre ofthe screen, but the origins ofthe displays ofV earlier derived sets aremoved back in correspeudence Y YVwith ships movements which haveoccurred since the deriving yof said earlier derived sets and prior tothe deriving of said last derived set. yIn this way, they veryadvantageous result is obtained that, although a display of thetruemotiontype is obtained, it differs from a true motion display asobtained by present commonly used radar systems in that it does notcreep towards the screen edge and therefore never requires re-setting. Afurther advantage is that the successive last sets of information, whicharemost persistentlyV and prominently displayed in the final display,show relative motions' of radio reiiecting targetssimultaneously with adisplay of true motion. f

It is not necessary to take off and display the sets ofinformationcyclically in order to produce a final true motion displayfor obviously this result can be achieved` if they be taken off anddisplayed (in said final display) simultaneously and this may be done,for example, in the last -part of the 21/2 second cycle of taking offand displaying hereinbefore mentioned. With such an arrangement there isobtained, on a single display surface, 'a

f speeded-up true motion display of reflecting targets, an

emphasised display of their last positions, and an early observeddisplay of their motions relative to the radar system and such anarrangement will be more fully described later with reference to FIG. v3of the drawing accompanying the complete specification.

The invention is illustrated. in the accompanying drawings. Itpthedrawingsth'e figuresl are numbered consecutively. FIGURE l showsschematically and in simplified diagrammatic manner a ship-borneembodiment by means of which therema'y be produced, at will, either atrue motion display or a display in which own ship appears fixed at thescreen centre of the' final display tube; FIG- URE 2 is a set ofrepresentations showing, diagrammatically, the nature of the displaysobtainablerfrom an equipment as illustrated in FIGURE l; and FIGURE 3shows, so far as is necessary to an understanding thereof, 'amodification.

' Referring to FIGURE l, a rotating azimuth scanning aerial system lrotated, for example, at 2O rpm; by a suitable motor drive (not shown)transmits radio pulses and receives retlected echo pulses, thetransmitted pulses being fed to the aerial system via a T-R cell 2 froma transmitter 3 and the received echo pulses being fed via said cell toa receiver 4. Output signals from the receiver 4 are fed as brighten-upsignals to what is herein termed the intermediate display cathode raytube 5, the screen of which has little or no afterglow. The ray in thetube 5 is subjected to radial deflection by a so-called range time baseand also to circular deflection at the same speed as the rotation of theaerial system l so that it produces a display of the P.P.I type. Anysuitable known means 6, controlled by the ships gyro compass, areprovided for compass stabilising the display on tube 5 so that the mostforward point of the display always corresponds to North direction. Themeans for deflecting the ray in the tube 5 are as well known and areaccordingly not shown in FIGURE 11. In fact the equipment as so farmentioned is that of a normal well known radar system giving a North-upcompass stabilised P.P.I. display and therefore requires no furtherdescription, its only difference from present day common practice lyingin the fact that the tube 5 is one having little or no afterglow insteadof being (as would be the case in a normal known equipment) a tube witha long afterglow screen.

Focused on the display produced on the screen of the tube 5 are threestorage type camera tubes C1, C2, C3 of the Perrnachon or similar typehaving a storage electrode structure which is scanned in television linefashion at suitable line and frame .frequencies by a pick-up cathode rayto develop output signals corresponding to the stored signals. Suchcamera tubes are well known and, as will be understood, each will giveoutput signals which, if fed to a television reproducer tube having acathode ray deflected in synchronism with the pick-up ray in the cameratube, would cause said tube to reproduce, in television fashion, thepictures on the screen of the tube 5 at whatever is the chosen framefrequency, e.g. 50 frames per second. In other words the cameras C1, C2and C3 view the screen of the tube 5 and produce television type outputsignals corresponding thereto. The

three camera tubes Cl, C2 and C3 are exposed successive.

ly (by means such as the three shutters 7, one for each camera tube,shown diagrammatically in FIGURE 1) at one minute intervals to onecomplete P.P.I scan (lasting approximately 3 seconds) on the screen oftube 5, each camera tube holding its own information for a period ofthree minutes at the end of which its stored signals are erased and anew picture stored. The means for controlling the shutters 7 and thesignal erasing means in an appropriate manner, being known to thoseskilled in the art, are not illustrated separately, being presumed to beincorporated in the representations C1, CZ and C3.

The signal outputs from the tubes Cl, C2 and C3 are fed to the statorsof three collector switches SC1, SC2, SC3. Each of these switches has arotor arm rotating round a stator consisting of three arcuate contactswhich are insulated from one another and each of which subtends an arcof a little less than 120. As will be clear from FIGURE l, each of thecamera tubes C1, C2, C3 feeds its output to three of the arcuatecontacts, one in each switch, the three contacts fed from any one cameratube being at 120 to one another and the three contacts of any switchbeing fed from a different camera tube. The three rotor arms of theswitches SCl, SC2 and SC3 are rotated together by a shaft S1 representedby a chain line (all mechanical drives are represented by chain lines inFIGURE l) at one revolution in 3 minutes. The drive to the shaft S1isshown as provided via a gearbox G by a motor M which is synchronisedto run at a suitable fixed speed relation with the motor (not shown)which rotates the aerial. In fact, if desired, the same motor whichrotates the aerial may be used to provide power for all the mechanicaldrives required.

d The three rotor arms of the switches SCI, SC2` and SC3 are connectedto the three insulated arcuate stator contacts of a further collectorswitch CD. Although thel stator contacts of this switch may be of equalangular extent, like those of the switches SCl, SC2, and SC3, it

is preferred to make them as shown, with two of them` each subtending atrifle less than 90 and the third sub- Y tending a trifle less than 180.The switch CD is driven at relative high speed, e.g. l revolution in21/2 "secs from the gearbox G by a shaft S2. As will be apparent thearrangement of the collector switches SCl, SCL-SC3 and CD is such that,during any one minute (the time between the deriving of successiveselected sets of information to be stored) the rotors of the switchesSCI, SC2 and SC3 will be feeding to the three different stator contactsof the switch CD, television signals corresponding to pictures producedon the screen of the tube 5 at the commencements of the last threesuccessive one minute periods, and in each 21/2 secs. the rotor ofswitch CD will take olf in succession from the three stator contacts thethree signals corresponding to the three pictures, two of which displaydata respectively 2 minutes and l minute earlier in time than thatdisplayed by the third. The long stator Contact the approximately 180contact-of switch CD isA the one connected to the camera tube (the tubeC3-of FIGURE 1 during the particular one minute phase of the threeminute cycle chosen for illustration) which is at the time consideredtelevising the latest picture. The signals on the rotor of switch CD arefed as brighten-up signals to the final reproducer cathode ray tube FRC,the ray in which is subjected to television type line and framedeflection (by means not separately shown) in synchronism with thecorresponding deflection of the pick-up rays in the camera tubes Cl, C2,C3,

The nature of the display by the nal tube FRC given by the apparatus ofFIGURE 1 as-so far described is indicated by diagrams (a) and (b) ofFIGURE 2 which are displays separated by a four minute interval for acase in which own ship is steaming North. In both diagrams the largespot A represents own ship and is fixed at the centre of the screen. Inboth figures the large spot B with the numeral 3 nearby, represents thelast relative position of another ship steaming West at the same speedas own ship and the smal-l spots 2 and 1 near the said largespotrrepresent that ships relative positions one minute and two minutes(respectively) earlier. As will be seen the other ship has a constantbearing of 045".` Each of the two pictures (a) and (b) shows at a glancethat the two ships are on a collision course and that own ship has theduty of keeping clear. The large i spot C with 3 nearby is the lastrelative position of a xed object, such as a buoy, the adjacent smallspots Zand 1l showing its relative positions one and two minutesearlier. Not the least of the advantages of the display given is that itgives an effect of accelerated motion i.e. although 5 the display at anyinstant shows what has occurred in two I 60 amar-95. v The displays of(a) and (b) of FIGURE 2 are relative motion displays. lf desired, theinvention may be used to give true motion final displays'eitherpermanently or at will. The remainder of the apparatus of FIGURE 1, now"to be described, will transform a relative motion display into a truemotion display. Referring again to FIGURE 1, L is a so-called log unitas known per se and adapted to give a D.C. voltage proportional to shipsspeed. This output is fed to a compass controlled resolver R, also asknown per se, and adapted to resolve the signal from L into twovectorially perpendicular components, one of which will be proportionalVnents obtained take'estimated drift into account.

to ships component speed North along a meridian and i and as well knownper se may be provided for inserting estimated tidal or. current speedso that the two compo- The meridianA component is fedvover threeresistors N1, N2, N3 to three condensers NKl, NKZ, NK3 which Aareaccordingly chargedat a rate proportional tothe ships speedkcomponent(which may, of course, be of either sign) alonga meridian. Similarly theparallel of latitude component (which may also be of either sign) isvfed through resistances E71, E2, E3 to charge three condensers EK1, EKZ,EK3 at a rate proportional to the ships speed component along a parallelof'latitude. Switches NSL NS2, NSS and E-Sl, ESZ and ESS are closed fora short v period of, say, l second at the commencement of each 1 minuteperiod, the closure beingreffected by a suitable mechanical drive Dactuated by a cam or like mechanisml (not separately shown) incorporatedin the gearbox G.v

The switches, when closed, connect the condensers NKI, NKZ, V NKS, EKl,EKZ, EK3 to charge smaller condensers-Nk1, Nk2, Nk3, Ekl, EkZ, Ek3respectivelysmall enough to charge up without seriously affecting thecharges in the larger condensers from which they are charged. The largercondensers NKLNKZ, and NK3 are successively discharged, atthecommencement of l minute intervals, by a three contact switch NDSAwhose rotor is driven by the shaft S1 while the condensers EKL EK2 kandEK3 are similarly successively discharged `by they switch EDS driven bythe shaft S1.

The live sides of the small condensers Nkl, NkZ and Nk3 are connected tothe stator contacts of three collector switches NDI, NDZ vandNDSv whichare like the shaft S1. Each of these condensers is connected to three-Acontacts at 120 one on each of the switches NDL'NDZ, NDS and each of thethree contactszof any one of these switches is connected to a differentoneof the three small condensers.V The small condensers Ek1Ek2 and Ek3are similarly connected with three collector switches EDI, EDZ and EDSalso driven by shaft S1'.

The rotors of the three switches NDI, NDZ and NDS are connected to thestator contacts of a collector switch NTM while those of the switchesEDI, EDZ and ED3 are connected'to the stator contacts Vof a collectorswitch ETM. `These two switches NTM and ETM have-each one large contactand two smaller ones and are like the switch'CD and are driven by thesame shaft S2. Their rotors are connected'to the input sides ofidentical ampliiiers NA and EA whose outputs are fed, via the two polesof a double pole switch RTM (when closed) to N-S and E-W deflectionshift coils schematically represented at NSk and EW and associated withthe final display tube FRC. *These coils are indicated as separate coilsbut they may, of course, beV the normally provided mutuallyperpendicular line and frame deflection coils of the tube. The sense ofthe windings is such that, when own ship has a positive (northerly)component of meridian velocity averaged over the interval between thederiving of any displayed earlier set of information and the deriving ofthe last set of information, the N-S deflection shift'coils apply asoutherly shift to the display of said earlier vset of storedinformation.V Similarly, a positive (easterly) component of averagedvelocity along a parallel produces a westerly shift.

`With this arrangement, when the switch RTM is closed,

Bearings measured between dots for the other ship and the correspondingdots for own ship--dot'1 from 1, 2'

from 2 and 3 fronrS-show at once that a collision course exists. In alldiagrams of FIGURE 2, the last position dots (3) at A, B and C aresmoreprolonged than the earlier position dots (indicated by showing the dotslarger) in each 21/2 sec. cycle because, in switches CD, NTM-and ETM onestator contact is larger than the other two. The emphasis thus placed onthe last position dots is of considerable operating advantage inpractice, Also it will be seen that, vin all four diagrams included inFIG. 2, the last positions of own ship (represented by dot A3) is alwaysat the centre of the tube screen.

As already stated it is not necessary, in order to obtain a true motioniinal display, to take off and display the sets of informationcyclically for it can be done Vsimultaneously. FIG. 3 shows how thearrangement of FIG. l may be modified to accomplish this. FIG. 3 showsonly the switches SCI, SC2 and SC3 of FIG. l and a switch CD whichreplaces theV switch CD of FIG. l. The rest of the arrangementillustrated by FIG. 3 is not shown because it is the same as in FIG. 1.the switch CD of FIG. 3 differs from the switch CD of FIG. l in havingtwo additional contacts each embracing a little 'less than 90 andspanning the second half of the long approximately 180 Contact. One ofthese extra contacts is connected to the rotor arm of SC1 and the otheris connected to the rotor arm of SC2. The rotor.

arm of CD has contacts sweeping over the various stator contacts asconventionally indicated. The result is to add a fourth phase to the21/2.second cycle-of rotation of CD', NTM and ETM in which theinformation stored in C1, C2 vand C3 is taken olf and displayedsirnultanemation and the number of stores of North and East motions ofown ship (three in the case illustrated) do not need to be the same asthe numbers of stores from which target information and motion signalsare takenv off andV displayed cyclically in each sequence of display.The

- number of stores must be at least two, is preferably not thedeflection shift coils will receive inputs representative i less thanthree, and may be more than three in cases in which it is important thatthe last stores of information should be brought up to date at as shortintervals as posv sible. Preferably the number of stores of each type isequal to or a whole multiple of the number of stores of each type fromwhich information and signals are taken oif anddisplayed cyclically.Thus the system of FIG. 1 could be extended to include twelve stores ofeach type, every fifth set of information being stored at intervals of15 seconds instead of every twentieth set at intervals of l minute, thenumber of stator contacts on each of switches on shaft S1 being, ofcourse, correspondingly increased to twelve. Moreover, the time taken bythe total switching cycle- 3 minutes in the case described withreference to FIGURE l-need not be constant but could be ar- 'ranged tobe increased for increased ranges for the purpose of obtaining displayedtarget tracks of useful length in the caseof targets which, being atlong range, give rise to target dots which move only slowly across theintermediate tube display screen. Various expedients as known per se inradar practice could be adopted if required. This, for example, amechanical drive from the ships gyro compass to the deflection coilassembly (including both scanning raster coils and true motion shiftcoils) on the final display tubercould be provided for the purpose ofmaintaining North on the tube screen As will be apparenty in alignmentwith Compass North and Ships head on said screen in alignment withactual ships head.

I claim:

1. A radar system suitable for marine use and comprising means forderiving, at a predetermined low periodicity, a succession of sets ofinformation, each set being of information as to the direction anddistance of radio reflecting targets within the range of the system; aplurality of information storage means; means for cyclically storing asequence of said derived sets in said information storage means with adifferent sequential set stored in each storage means; means for takingoff, at a relatively high cyclic speed of repetition which is highrelative to the first-mentioned low periodicity, the sets of informationstored at any given time in said informah tion storage means; signalstoring means for storing, during the deriving of the succession of setsof information, signals representative of the amount and direction ofmovement of the radar system as a whole during the time taken to derivesaid succession of sets, means for utilizing the sets of informationtaken off to produce a final display of the HP1. type, and further meansfor utilizing the stored movement representing signals to deect theorigin of the P.P.I. display of each set of information in said finaldisplay on the surface on which said nal display is produced by anamount representative of the amount of and in a direction opposite tothe direction of said movement of the radar system as a whole during thetime elapsing between the deriving of the set in question and thederiving of the last set to be derived so that the final display is atrue motion display in which the last position, at any time, of theradar system as a whole is always located at a predetermined position onthe surface on which the nal display is produced.

2. A radar system as claimed in claim 1 wherein the time during whichstored sets of information are taken off from the plurality ofinformation storage means is not the same for each information storagemeans, the time occupied in taking oif from and displaying a set ofinformation stored in that information storage means storing the mostrecently derived set being longer than the time occupied in taking oiffrom and displaying a set of information stored in any other informationstorage means.

3` A radar system as claimed in claim 2 wherein there are threeinformation storage means employed to store three sets of informationderived during three successive periods of time and approximately onehalf the complete cycle of time during which the stored sets ofinformation are taken off from the said three storage means is occupiedin taking oir" information from the storage means storing the latest ofsaid sets.

4. A radar system as claimed in claim 1, wherein said informationderiving means comprising an azimuth scanning aerial, a pulsed radartransmitter, an echo signal receiver, an intermediate PPI. display tubeof little or no afterglow, the ray in which is deected radially underthe control of a range time base and circularly at the speed of aerialscanning in azimuth, and means for compass-stabilising the saidintermediate P.P.I. display; wherein a periodic succession of selectedsets of information occurring at predetermined long intervals are storedby said information storage means which include a plurality of cameratubes focused on the successively eX- posed each to a different one ofthe selected pictures produced on the screen of the intermediate displaytube; and wherein signals from the camera tubes are fed at apredetermined high speed of repetition to said information utilizingmeans which comprises a final display tube adapted and arranged toproduce the P.P.I. final display.

5. A radar system as claimed in claim 4 wherein the l@ P.P,I. display inthe final display tube is produced by a television scan.

6. A radar system suitable for marine use and comprising means forderiving, at a predetermined low periodicity, a succession of sets ofinformation, each set being of information as to the direction anddistance of radio reiiecting targets within the range of the system; aplurality of information storage means; means for cyclically storing asequence of said derived sets in said information storage means with adifferent sequential set stored in each storage means; means for takingoff, at a cyclic speed of repetition which is high relative to thefirst-mentioned low periodicity, the sets of information stored at anygiven time in said information storage means; signal storing means forstoring, during the deriving of the succession of sets of information,signals representative of the amount and direction of movement of theradar system as a whole during the time taken to derive said successionof sets; and means including a final display cathode ray tube forcombining said sets of information taken otf and the stored movementrepresenting signals to produce a reversed true motion display on thescreen of said final display tube, wherein said reversed true motiondisplay comprises a plurality of P.P.I. pictures, each corresponding toa different one of said derived sets of information, with the origin ofthe picture of the last derived set of information being located at afixed predetermined point on said screen and the origins of the otherpictures displayed at any given time being spaced from saidpredetermined point by respective amounts corresponding to and indirections opposite to the movement of the radar system during the timeintervals required to derive the particular sets of information beingdisplayed.

7. A radar system carried on board ship and comprising means forderiving, at a predetermined low periodicity, a succession of sets ofinformation, each set being of information as to the direction anddistance of radio refleeting targets within the range of the system; aplurality of information storage means; means for cyclically storing asequence of said derived sets in said information storage means with adifferent sequential set stored in each storage means; means for takingoit, at a cyclic speed of repetition which is high relative to thefirst-mentioned low periodicity, the sets of information stored at anygiven time in said information storage means; means for producing DC.signals representative of mutually perpendicular components of shipsmotion during the deriving of the succession of sets of information;condenser storage means for storing said D.C. signals; means including afinal display cathode ray tube for utilizing the sets of informationtaken off to produce a nal display of the RPI. type; and means forutilizing the D.C. signals stored in said condenser storage means forshifting the origin of the display in the final display tube incorresponding perpendicular component directions of such sense that theorigin of the display of the last derived stored set of information isalways located at the center of the screen, but the origins of thedisplays of earlier derived sets are moved back in correspondence withships movements which have occurred since the deriving of said earlierderived sets and prior to the deriving of said last derived set.

References Cited by the Examiner UNITED STATES PATENTS 2,716,203 8/55Sen et a1. 343--11 2,822,536 2/58 Sandretto 343-11 2,956,274 10/ 60Smythe 343-11 CHESTER L. JUSTUS, Primary Examiner.

1. A RADAR SYSTEM SUITABLE FOR MARINE USE AND COMPRISING MEANS FORDERIVING, AT A PREDETERMINED LOW PERIODICITY, A SUCCESSION OF SETS OFINFORMATION, EACH SET BEING OF INFORMATION AS TO THE DIRECTION ANDDISTANCE OF RADIO REFLECTING TARGETS WITHIN THE RANGE OF THE SYSTEM; APLURALITY OF INFORMATION STORAGE MEANS; MEANS FOR CYCLICALLY STORING ASEQUENCE OF SAID DERIVED SETS IN SAID INFORMATION STORAGE MEANS WITH ADIFFERENT SEQUENTIAL SET STORED IN EACH STORAGE MEANS; MEANS FOR TAKINGOFF, AT A RELATIVELY HIGH CYCLIC SPEED OF REPETITION WHICH IS HIGHRELATIVE TO THE FIRST-MENTIONED LOW PERIODICITY, THE SETS OF INFORMATIONSTORED AT ANY GIVEN TIME IN SAID INFORMATION STORAGE MEANS; SIGNALSTORING MEANS FOR STORING, DURING THE DERIVING OF THE SUCCESSION OF SETSOF INFORMATION, SIGNALS REPRESENTATIVE OF THE AMOUNT AND DIRECTION OFMOVEMENT OF THE RADAR SYSTEM AS A WHOLE DURING THE TIME TAKEN TO DERIVESAID SUCCESSION OF SETS, MEANS FOR UTILIZING THE SETS OF INFORMATIONTAKEN OFF TO PRODUCE A FINAL DISPLAY OF THE P.P.I. TYPE, AND FURTHERMEANS FOR UTILIZING THE STORED MOVEMENT REPRESENTING SIGNALS TO DEFLECTTHE ORIGIN OF THE P.P.I. DISPLAY OF EACH SET OF INFORMATION IN SAIDDISPLAY ON THE SURFACE ON WHICH SAID FINAL DISPLAY IS PRODUCED BY ANAMOUNT REPRESENTATIVE OF THE AMOUNT OF AND IN A DIRECTION OPPOSITE TOTHE DIRECTION OF SAID MOVEMENT OF THE RADAR SYSTEM AS A WHOLE DURING THETIME ELAPSING BETWEEN THE DERIVING OF THE SET IN QUESTION AND THEDERIVING OF THE LAST SET TO BE DERIVED SO THAT THE FINAL DISPLAY IS ATRUE MOTION DISPLAY IN WHICH THE LAST POSITION, AT ANY TIME, OF THERADAR SYSTEM AS AS WHOLE IS ALWAYS LOCATED AT A PREDETERMINED POSITIONON THE SURFACE ON WHICH THE FINAL DISPLAY IS PRODUCED.